Alkali metal vapor resistant glass



Patented Feb. 11, 1936' 1 UNITED STATES PATENT OFFICE ALKALI METAL VAPOR RESISTANT GLASS Marcello Pirani, Berlin-Wilmersdorf, and Georg Gaidies, Berlin-Pankow, Germany, assignors to General Electric Company, a corporation of New York 3 Claims.

The present invention relates to glass compositions generally and more particularly the invention relates to such compositions which are resistant to the chemical effects of alkali metal 5 vapor.

This application is a division of our co-pending application, Serial No. 592,137, filed February 10, 1932. In another co-pending application, Serial No. 470,436, filed July 24, 1930 we have described an alkali metal vapor resistant boro-silicate glass for use in the container of an electric discharge device having a filling of alkali metal vapor and particularly sodium vapor. Such glass contains in addition to the usual constituents of glass, as alkalies, alkali earths and aluminum oxide, not more than 50 per cent of silica and not less than boric oxide. The following formula was given for the production of a suitable glass:

Per cent NazO 4 to 5 A1203 11 to 13 CaO 10 to 11 B203 40 to 60 SiOz 10 to 26 and the following composition was given for a particularly good glass:

In making the above glasses the aluminum oxide has been added heretofore, as is customary in the glass industry, in the form of alumina, and the silica has been added heretofore in the form of commercial glass sand.

We have discovered that when such commercial materials are used in the production of alkali vapor resistant glass made in accordance with the above formula and having the above composition the container of an electric discharge device made of such glass and having an alkali metal vapor filling, while having sufficient chemical and heat resistivity for such purpose, has a tendency toward discoloration of the container walls after the electric discharge device has had a comparatively long operating life, this being true especially in containers having a small cross section.

The object of the present invention is to decrease the rate at which this discoloration of the walls of such a container of such an electric discharge device takes place. and advantages attaching to the invention will be apparent to those skilled in the art from the following particular description.

The invention attains its object by using chemically pure silica and aluminum oxide in the pro- 10 duction of alkali vapor resisting glass made in accordance with the formula given above in place of the commercial glass sand and alumina used heretofore. We have discovered that such commercial substances contain more or less iron ox- 15 ide and this material incorporated in the walls of the container produces a discoloration thereof after the device has been operating for an appreciable time. We have discovered in addition that the iron oxide content of the glass mixture or of 20 the molten glass mass must not exceed 0.1 per cent part by weight if discoloration of the walls of the container of the device is to be lessened during the operation of the device.

When desired, materials to effect the evapora- 25 tion of the iron oxide, such as sodium chloride, is added to the molten mass of the glass batch to reduce the iron oxide content thereof instead of using chemically pure materials for such glass.

As the glass disclosed above is slightly hygro- 30 scopic it is desirable that the container of an electric discharge device made of such glass be .protected from the surrounding atmosphere by a covering or casing made of a non-hygroscopic glass well known in the art, such as Thiiringer glass, which has a better acid, lye and water stability than the boro-sili'cate glass of the present invention. 'I'hiiringer glass and the borosilicate glass of this invention have approximately the same coeificients of expansion and are capable 40 of being fused or fritted together to form a single walled two-layer container where that is desired. While we have described and set forth processes for producing our improved product, it is obvious that various changes may be made in the processes or in the separate steps thereof without modifying or changing the essential features and characteristics of the product produced and that such product remains substantially the same.

Still further objects 5 What we claim as new and desire to secure by Letters Patent of the United States is:-

V 1. A new article of manufacture being a glass resistant to the chemical effects of alkali metal vapor and consisting of alkalies, alkali earths, aluminium oxide, not less than 30% boric oxide and not more than 50% silica and containing not more than 0.1% iron oxide.

2. A new article of manufacture being a glass '10 made. in accordance with the following formula:

MARCELLO PIRANI. GEORG GAIDIES. 

